A series of investigations of variables suspected to influence the development of generative language learning abilities among mentally retarded persons is proposed. Specifically, the focus of inquiry is on learning processes responsible for observational learning of linguistic structures (selective imitation), for transfer between receptive and expressive language modalities, and for recombinative generalization (the application of syntactic rules) within both modalities. Single subject research designs will be employed to determine how the environment may be structured in order to provide language exposure in ways that contribute to efficient and generalized learning by mentally retarded individuals. Approximately 18 moderately and severely mentally retarded children (5-18 years) whose language expression ranges from one to four morphemes per utterance will participate in each of three years of this project. The research will be conducted at a public school center for the handicapped where they are enrolled. First, lexical acquisition will be investigated. The comprehension of new object words will be taught using receptive work-referent discrimination training and observational learning procedures. Functional relationships between training procedures and the establishment of receptive and expressive responding will be investigated. Second, syntactic acquisition will be studied. Miniature linguistic systems comprised of words from different semantic classes, recombined according to specific syntactic (word order) rules, will be taught using both discrimination training and modeling procedures in order to establish recombinative generalization in the receptive and expressive language modalities. Tests for transfer of acquired knowledge to untrained tasks requiring receptive performance, expressive performance, and transfer to other school environments will be included. The planned research has implications for psycholinguistic theory in general, and for developing a functional analysis model of language acquisition in particular. The research also is significant in its direct contribution to the development of more efficient language intervention procedures than can be employed with developmentally disabled individuals.